1000 W Amplifier for S-band Linear Accelerator Applications

The S-band has long been an application space servicing both commercial and military radars and now supports communications (WiMAX and cordless telephony) and ISM (microwave ovens and linear accelerators). The IBA2856S1KW from Integra is the first amplifier designed for RF pulsed power for S-band linear accelerator or linac applications. The current technologies established by the existing S-band linear accelerators include tube transistors (TWT), most often high power klystrons. The pulsed klystrons produce extreme amounts of pulsed power often in the megawatts range yet are difficult to manufacture and with few manufacturers are getting hard to come by. The industry will continue to use klystrons for the MW final output stage; these klystrons require a 1 KW driver. There is a market for a 1 KW driver solution using highly reliable and cost-effective solid-state technology as TWT replacements.

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Integra Technologies has designed a multiple stage line-up that produces more than 35 dB of gain across the band at full RF output power. The IBA2856S1KW amplifier is characterized at center frequency 2.856 GHz and exhibits ± 20 MHz bandwidth. The amplifier is characterized over the entire bandwidth with a strict droop specification of 0.8 dB. The rise and fall times are both below 100 ns.

Several transistors are combined in parallel of the final output stage to produce a minimum of 1000 W of pulsed RF output power. The amplifier is able to produce 1200 W of pulsed RF output power, which is needed when the gain of the klystron is low. However, in many instances only 1000 W is required and the extra headroom in performance translates to the reliability of the amplifier as the transistors are not working very hard to produce the 1 KW of power.

Integra offers two discrete bipolar devices: the IB2856S30 (30 W output power device used as a driver) and the IB2856S250 (250 W output power device used as a final device) characterized at spot frequency 2.856 GHz and pulse conditions (12 us pulse width and 3 percent duty cycle) for S-band RF electron beam guns. Using these two devices, Integra leveraged multiple technologies including LDMOS, used in the pre-driver and driver stages, to produce higher gain than the bipolar technology can support. Integra utilizes multiple technologies in combination with the design of the pallets, which results in an optimum amplifier for RF power sources within linear accelerators.

Figure 1 IBA2856S1KW output power vs. frequency.

The IBA2856S1KW is an easy to use, four-stage, 1000 W solid-state amplifier for S-band applications. Utilizing a combination of LDMOS and bipolar technology achieves over 35 dB of gain while delivering more than 1 KW of RF pulsed output power across the frequency band of operation from 2.836 to 2.876 GHz. The devices are enclosed within a chassis with dimensions 8" × 12" × 1.25" and mounted on an aluminum heatsink for excellent thermal performance and reliability. Both the input and output connectors are of the SMA type and the DC voltage is both 28 and 40 V. As reliability is a chief concern, the output is protected with a RF circulator. The amplifier is capable of handling a maximum of 12 A of peak current.

Figure 2 IBA2856S1KW gain and input return loss vs. frequency.

RF characterization of the IBA2856S1KW amplifier was conducted with 28 V of drain bias on the LDMOS driver stages and 40 V of collector bias on the bipolar final stages. The amplifier was characterized with pulsed input signal conditions with a 12 μs pulse width and 3 percent duty cycle. The electrical performance of the IBA2856S1KW is shown in Figure 1. The curve shows output power performance exceeding 1000 W across the frequency band of operation between 2.836 and 2.876 GHz. Figure 2 displays the power gain and input return loss figures for the amplifier. The power gain exceeds 35 dB and the IRL better than 17 dB across the operating frequency band.